Modeling the anisotropic elastocaloric effect of textured NiMnGa ferromagnetic shape memory alloys

Directionally solidified NiMnGa ferromagnetic shape memory alloys (FSMAs) can exhibit anisotropic elastocaloric effect due to large transformation latent heat release/absorption and strong initial texture. Nowadays, these alloys are promising candidates for the core components of solid-state refrigeration devices. In this work, based on crystal plasticity theory, a constitutive model is constructed to predict such anisotropic elastocaloric effect. Firstly, a thermo-mechanically coupled constitutive model is constructed at single crystal scale to describe the non-isothermal stress-induced transformation between austenite phase and modulated martensite one with a periodicity of 5 lattice cells (5M). Internal heat generation originating from transformation latent heat and irreversible mechanical dissipation is considered. A thermo-mechanically coupled self-consistent homogenization scheme is developed to estimate the interaction among grains and the heat exchange with environment, and calculate the overall deformation and temperature variation of the polycrystalline aggregates. By introducing a strong {100}_A initial texture, the established model is applied to analyze the anisotropic elastocaloric effect of textured polycrystalline NiMnGa FSMA. It is found that the quantitative results obtained in experiments such as stress-strain response and adiabatic temperature change can be reasonably captured by the proposed model. Furthermore, the isotropic elastocaloric effect in the un-textured NiMnGa FSMAs is predicted. It is shown that the initial texture plays an important role on the elastocaloric effect, and the textured NiMnGa FSMAs have obvious advantages over the untextured ones. The proposed model provides a theoretical guidance for the design of solid-state refrigeration devices at the micro- and macro-scales.